Communication systems utilize various modulation schemes depending on the particular medium, bandwidth, and information signal. To increase spectral efficiency, many communication systems utilize multi-level modulation schemes. For example, wireless, copper-based, and other communication systems can utilize orthogonal frequency-division multiplexing (OFDM), quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), polarization multiplexing, and the like. Each of these modulation schemes transmits multiple bits per symbol. For example, QPSK scheme is attractive as it transmits two bits per symbol, thereby reducing signal baud (i.e. symbol) rate by a factor of two.
With regard to optical communication systems, high data rate signals, such as 10 Gb/s and 40 Gb/s, are conventionally used. These utilize a binary modulation scheme, e.g. on-off keying. Disadvantageously, binary modulation schemes provide a very poor spectral efficiency and limit overall transmission system utilization in wavelength division multiplexed (WDM) networks.
Continuing rapid growth in network bandwidth requirements is pushing single channel data rates towards ever increasing speeds. For example, current standards bodies are pursuing data rates of 100 Gb/s, which would require even higher transmission rates (i.e. ˜112 Gb/s), once FEC and Framing overheads are considered. Other standards are considering data rates of 120 Gb/s, again requiring even higher transmission rate (i.e. ˜130 Gb/s) once FEC and Framing overhead is added. Disadvantageously, such high data rates are beyond current limits of the electronics and optics using a direct binary modulation scheme.
Accordingly, optical communication systems are moving towards multi-level modulation schemes to improve spectral efficiency and to reduce the demands on the system electronics and optics. In particular, a Differential-QPSK (DQPSK) scheme is attractive as it transmits two bits per symbol, thereby reducing signal baud (i.e. symbol) rate by a factor of two. At the same time, tolerance to chromatic dispersion is increased and corresponds to the reduced baud rate (not bit rate) of the signal. Additionally, demands on the bandwidth of electronic components are also reduced corresponding to the baud rate, and not bit rate of the signal. Multi-level amplitude modulation is also possible for optical communication systems, but is not very attractive for optical communication as it does not provide a high separation between adjacent levels, and is susceptible to noise. A combination of both Amplitude and Phase modulation is also possible, and can be generalized as optical QAM modulation scheme.
A feature common to all multi-level modulation schemes is the splitting of the incoming data bit stream into sub-rate (baud-rate) data stream for subsequent modulation, transmission, and reception. At the same time, data processing, such as FEC encoding and decoding, is advantageously performed on the full bit rate signal. This arises due to the fact that sub-rate data streams driving optical modulator are correlated, and not independent. Hence, whatever additional bit manipulation (i.e. FEC) occurs, it must happen before the parallel sub-rate driving signals are generated.
Disadvantageously, multi-level modulation schemes, such as DQPSK, require a more complex receiver design. Generally, optical multi-level receivers require an ability to dynamically control various demodulation blocks, such as optical and electrical demodulation blocks. For example, these receivers also generally require tunable optical chromatic and polarization-mode dispersion compensation for high-data rate signals. Typically, this compensation is applied to a composite signal at full bit-rate. Other demodulation blocks, such as Delay Line Interferometers (DLI) as are required for DQPSK modulation formats, are applied to individual sub-rates (i.e., symbol) signals. Multi-level amplitude modulation formats require threshold-based level differentiation applied to the composite signal. Receiver-based electronic distortion compensation, as well as amplitude and phase decision thresholds can also be applied to the individual sub-rate signals.
Depending on the particulars of the demodulation block and modulation format, dynamic control of various settings can be accomplished by signal monitoring mechanisms that look at some easily measured property. For example, tunable dispersion compensation (TDC) and polarization mode dispersion compensation (PMDC) blocks can be controlled by measuring radio frequency (RF) spectrum shape of the received signal, or some property related to the clock tones. PMDC can be further controlled based on Degree of Polarization measurement, or some other optical monitoring scheme as is known in the art. DLIs can be set based on relative powers of the dual outputs.
The goal of stabilization and dynamic control schemes is to achieve the lowest possible Bit Error Rate (BER) of the composite signal. As such, any indirect scheme that relies on optical, RF, power, or some other monitoring is only indirectly related to the BER. Such schemes can only achieve a coarse accuracy, and are susceptible to errors caused by multiple distortion effects that could be present at the same time.
Thus, a stabilization and dynamic control scheme is needed that links control directly to the BER of the received signal, allows for efficient start-up tuning, allows for fast and efficient operational tracking, and allows for independent BER monitoring and control of sub-rate components. Start-up conditions are especially onerous, as parameters can be so far from optimum as not to allow FEC frame locking and exhibit a corresponding infinite BER. The problem becomes a multi-dimensional blind parameter search until signal quality is sufficiently high to allow FEC framer locking. Existing schemes based on a composite signal BER monitoring are difficult to apply in cases where multiple parameters require tracking as well.